"I feel that the best way I can be of service at this time is to continue to work for the health and wellbeing of the people of Hawaii and our country in Congress, and to stand ready to serve in uniform should the Hawaii National Guard be activated," she said.

The Republican choice will almost certainly be President Donald Trump, who has overwhelmingly won the first two contests.

For a Democratic field that was once the most diverse in history, the debate stage will be the starkest example yet of the challenges facing candidates of colour.

Public opinion polls show Americans in general are opposed to U.S. military interventions overseas.

Two Democrats voted against impeaching Trump, while another voted for just on article. Another Democrat was not present for the votes and a presidential candidate voted 'present.'

Wednesday’s tussle began with squabbles over a program that some Democrats worry could alienate swing voters who are wary of fully government-run health care and fear it would be extraordinarily difficult to get through Congress.

The debate also touched on health care and gun control over a three-hour forum in Westerville, Ohio

Ten Democrats railed against a national economy and Republican administration they said exists only for the rich as presidential candidates debated onstage for the first time in the young 2020 season.

Gabbard has not been a factor in early 2020 opinion polls, and her nascent campaign already has shown signs of trouble.

Castro said he was running for president "because it's time for new leadership, because it's time for new energy and it's time for a new commitment to make sure that the opportunities that I've had are available to every American."